This invention relates to the pelletization of polymers, and more specifically to the design of dies for underwater pelletization of high melt flow rate polymers.
High melt flow rate polymers are very fluid during extrusion and are frequently extruded and pelletized under water. Underwater pelletization provides for sufficient surface cooling of the individual pellets to prevent agglomeration.
Underwater pelletization is typically used for polyethylenes having a melt index (MI) from 0.1 to 500 (ASTM D1238, Condition E) and for polypropylenes having a melt flow rate (MFR) from 0.5 to 100 (ASTM D1238, Condition L). Dies for underwater pelletization of polymers have a plurality of orifices arranged on an annular cutting surface such that rotating knife blades can cut the polymer strands shortly after the polymer is extruded from the orifices. The shape of the orifices in a conventional die generally consists of a circular hole with an initial large diamter preland and a narrower diameter final hole. A gradual transition from the preland to the final hole is typically incorporated to prevent buildup of hardened polymer in the orifice.
The production of ultra high melt flow rate (UHMFR) grade polymers is difficult because of pelletizing problems. A UHMFR polymer has a MFR of about 50 or greater. The MFR of a UHMFR polymer can be as high as 2000 or greater. UHMFR polymers are particularly useful for the production of non-woven fabrics by melt blown fiber processes. To be usefully employed in commercial processing equipment the UHMFR polymer must first be prepared as a pellet feed stock. Polypropylenes having a MFR above about 40 are particularly difficult to pelletize.
Attempts to pelletize UHMFR polypropylenes with conventional underwater pelletization systems result in an excess amount of non-uniform pellets, malformed pellets including tailed pellets, long pellets, elbows, dog bones, and pellet trash. Malformed and non-uniform pellets are undesirable since they tend to bridge in pellet feed hoppers and to block pellet conveying systems. Further, significant amounts of malformed pellets alter the bulk density of the pellet stock which causes feeding problems in the extrusion line and which may result in voids in the final product. In addition to malformed pellets, trashouts occur frequently during production of ultra high MFR polypropylene. Trashouts are extruder shutdowns resulting from polymer buildup on the rotating knives.
Ultra high melt flow rate polypropylene pellets have been produced with fewer malformed pellets by a process known as strand chopping. For strand chopping, the molten polymer strands are extruded into a water trough having little or no currents and eddies. The cooled strands are then passed through a strand chopper which cuts the strands into cylindrical pellets. Although spherical pellets would be more uniform, the cylindrical pellets are known to have fewer malformed pellets, and thus fewer problems with plugging of pellet conveying equipment and with measuring polymer feed by volume. Strand chopping pelletization units are limited to low throughputs because it is difficult to keep the strands separated. Therefore, it is desirable to develop technology for underwater pelletizing of high MFR polymers.